STEEL 



COLLIER 



35 CENTS 







































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COPYRIGHT, 1910 
BY 

ARTHUR L. COLLIER 






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©CIA271465 





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V 


PREFACE 


I have written this brief treatise on steel, believing that 
such an article would appeal to many of the daily workers in 
this metal who desire to better inform themselves of its nature. 

The first part concerns the principle methods by which 
steel is made; the second its structure and characteristics under 
different conditions. 

Preceding the subject in hand are given a few definitions 
of certain terms used in the book. 

Arthur L. Collier. 


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DEFINITIONS 


Allotropic Forms: The different modifications in which an 
element may occur. 

Example : Diamond, charcoal, graphite, forms of carbon 
Alpha, beta, and gamma iron: forms of iron. 

Alternate Stress: That stress produced in a body by bending 
it back and forth so that its fibres are alternately under 
tension and compression. 

Garburizer: A substance, usually spiegeleisen or ferro-man- 
ganese, added to molten steel for the purpose of increasing 
the carbon content and of purifying the metal. 

Chemical Symbol: An abbreviation, that is used to denote 
elements or compounds. 

Example : Manganese=Mn; Manganese sulphide=MnS. 

Compound : A chemical combination of two or more elements. 
Example : Iron carbide, manganese sulphide. 

Elastic Limit: The number of pounds per square inch of 
cross section at right angles to the strain, at which stress 
ceases to be proportional to the strain. 

Element: A substance which has never been decomposed. 
Example: Iron, Fe; Carbon, C; Maganese, Mn. 

Elongation: Generally taken as the percentage of stretch 
in a specimen two or eight inches long when the stress is 
the ultimate tensile stress. 

Note: This foims a basis for comparing static ductility 
or toughness. 

Latent Heat: The heat that produces change of state in a 
substance without raising its temperature. 

Note: When a substance thus changed reverts to its orig¬ 
inal form this latent heat is given up. 

Oxide: r A substance combined with oxygen. 

Example: Silicon dioxide, Si 0 2 ; Manganese oxide, Mn O; 
Iron oxide, FeO. 


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NOTE: Some oxides, such as the first, act as acids; others, 
such as the second, and third, act as bases. The first 
combines with the second or third, forming in the one 
case a silicate of manganese ( MnSi0 3 ) and in the 
other ferrous silicate (FeSi0 3 ). Both these sub¬ 
stances are components of the slag. 

Oxidation: The chemical action, whereby oxygen is added to 
a substance. 

Example : Carbon (O + Oxygen (O'! = Carbonic Oxide 
(CO). 

Saturated Solution: A solution in which the solvent is hold¬ 
ing all the solute that it can under the existing conditions. 
Example : Molten steel containing about 2 per cent of 
carbon. 

Slag: A molten mass of impurities formed in smelting or re¬ 
fining. 

NOTE: In iron metallurgy the silicates of iron and man¬ 
ganese dissolve in each other forming part of the slag. 
This mass, in turn, dissolves other oxidized impurities. 

Solution: A chemical union of two or more substances, some 
of which are dissolved in some of the others. 

Example: Molten steel. 

Solute: The substance dissolved by the solvent. 

Example: The carbon of molten steel. 

Solvent: The substance of the solution which does the dis¬ 
solving. 

Example: The iron of molten steel. 

Reduction: The chemical action whereby substances are 
broken up into elements or other compounds. 

Example: Silicon ( Si)+ Ferrous Oxide ( 2FeO'> = Silicon 
dioxide ( Si0 3 ) + Iron ( Fe ). 

NOTE: Here ferrous oxide (FeO'l is reduced and silicon 
( Si) is oxidized. 

Reduction of Area: A phrase, representing the amount that 
the cross section of the specimen is reduced as compared 
with the original area; the measurement being taken at 
that point where the bar “ necks ” in breaking. 

Note: This forms a basis for comparing static ductility 
or toughness. 

Repeated Stress: A stress repeatedly applied and relieved 
with short intervals between applications. 

Tensile Strength: The number of pounds per square inch 
of cross section necessary to pull the test specimen apart. 


4 


THE MANUFACTURE OF STEEL 


The products of iron so blend with each other, that it 
becomes most difficult to differentiate between them and speci¬ 
fically state that at such a point wrought iron ceases to be 
wrought iron and becomes steel; or that at such a point steel 
ceases to be steel and becomes cast iron. Instead, it is much 
easier and more in keeping with practical nomenclature, to 
classify these substances according to the methods by which 
they are made. Thus steel is generally considered to be that 
product which is produced by the process of cementation, 
or the malleable compounds of iron made in the crucible, the 
converter, or the open hearth furnace. From a chemical 
standpoint wrought and cast iron are equally worthy of the 
name steel since all these products contain the same elements. 
If the elements such as silicon, manganese, sulphur, phosphorous, 
etc., are considered the impurities of iron, then wrought iron, 
containing less of these, would be a pure form of steel; and cast 
iron, containing more of them, would be an impure steel; while 
the products made by the methods enumerated above, would 
be those of steel lying between these two in purity. 

Pig iron, or cast iron as it is sometimes called, is the product 
of the blast furnace. It is made by reducing the iron ore with 
fuel, which is ignited by a blast of air at high temperature. 
The fuel is usually coke; though sometimes charcoal or an¬ 
thracite or a mixture of these fuels is used. Limestone is 
also charged with the ore and fuel to serve as a flux in the melting 
zone of the furnace. By dissolving what impurities are oxidized, 
and by mixing with the ash of the fuel and the gangue of the 
ore, a fusible slag is formed, which, being lighter than the 
molten iron, floats on top of it, and thus is readily withdrawn 
and disposed of separately. These three substances, namely 
ore, coke and limestone, are charged in alternate layers through 
a hopper at the top of the furnace, and mechanical means are 
now almost in entire use at the blast furnace works for this and 
other operations, because of the great size that they now build 
these furnaces, it being not uncommon to find them of a height 
of from ninety to a hundred feet. The air, of which some 
25,000 cubic feet per minute is required for the larger furnaces, 
is blown first through large upright cylinders containing pre- 

5 


heated firebrick, which heats it, and then into the furnace 
through tubes called tuyeres, where it strikes the white hot 
coke at a point about ten feet above the level of the hearth, 
and oxidizes most of the carbon of the fuel forming carbonic 
oxide gas, which immediately rises, and passing up through the 
charge evolves different chemical reactions, until it with other 
gases formed pass off at the top of the furnace, thereafter to 
be used as fuel under boilers, or to be cleansed for use in the 
gas engines. 

Thus the charge at the top becomes well heated, and as it 
gradually moves down through the furnace it gains in tempera¬ 
ture, while chemical reaction becomes more energetic. Half 
way down the furnace, where the temperature is about 1470°F, 
the ore becomes reduced, and the iron formed drips down 
through the charge, absorbing carbon as it goes. It finally 
collects in a molten mass on the hearth, and is tapped or run 
from a hole near the bottom. The metal is run into sand 
moulds on the floor of the casting room, or into pig casting 
machines, where it soon solidifies. The fusible slag is tapped 
through a separate hole in the side of the furnace, run through 
a spout into cars and hauled to the slag dump. The oxidized 
substances pass into the slag, and the reduced substances 
pass into the iron on the hearth. It is for this reason that 
iron runs high in the reduced elements such as silicon, man¬ 
ganese, sulphur and phosphorous. This last element it is 
especially difficult to fuse into the slag, because the others are 
much more active chemically, and tend to displace it under the 
conditions that exist in the furnace. Thus low phosphorous 
ores are the ones most used, if a low content of this element 
is desired in the final product. 

One of the oldest known processes for making steel is that 
in which the metal is melted and purified in a crucible. It is 
a method much in use to-day for our finer grades of tool steel. 
Small pieces of wrought iron, or wrought iron mixed with 
scrap, are put in graphite crucibles; and on top of this charge 
are placed charcoal, togjther with the other alloying elements 
desired, «uch as tungsten, manganese, etc., as well as ingre¬ 
dients to make the slag fluid. After sealing the crucible it 
is placed in a furnace heated to high temperature, and there 
left until the metal is “ dead.” This is the point at which the 
metal will pour without evolution of gases. The “ deadening ” 
is brought about by the absorption of silicon from the walls 
of the crucible, or from the cast iron, -which is sometimes used 
in place of charcoal as a carburizer. This absorption of silicon 
deoxidizes the metal, and thereby strengthens it, but if too 
much is taken up it tends to weaken it. The steel, in this 

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process, gets its carbon, manganese, and silicon by absorption 
from the ingredients put on top, and though it is the intention 
to exclude the elements phosphorous and sulphur, yet these 
get into the finished material to small extent; the former from 
the slag in the wrought iron, and the latter from the walls of 
the crucible. 

The next important process, and one named after its 
inventor, is the Bessemer. In this method a large vessel swung 
on trunnions is used, and into it is poured the molten metal, 
which may come direct from the blast furnace or a large re¬ 
ceptacle known as a mixer. The process is then known as the 
direct process. Or the molten metal may come from a cupola. 
The process is then known as the indirect process. In either 
case the purification of this molten cast iron is accomplished 
by blowing air through holes in the bottom of the vessel and 
thence through the molten iron; thus oxidizing the impurities 
which in the form of a slag float on the surface. Silicon and 
manganese having the greatest affinity for oxygen under the 
prevailing conditions, are first burned out; and the combustion 
of these elements generates heat, and raises the temperature 
of the mass. Three or four minutes after the blast is turned on 
and carbon in its turn is oxidized. From now on, until the 
carbon is burnt out there will play from the mouth of the vessel, 
or converter as it is better known, a long flame called the carbon 
flame, which is, in reality, a flame of carbonic oxide gas. The 
drop of this flame at the end of nine or ten minutes is a sign, 
that the metal is decarburized to a percentage of not over 
.3 carbon. The converter is then turned down and the blast 
shut off. Phosphorous and sulphur are not eliminated in this 
process, because the compounds formed cannot be dissolved 
in the slag, owing to its greater attraction for the oxides of silicon 
and manganese, which it dissolves to the exclusion of the others. 
Hence the necessity of a pig iron low in phosphorous, if the same 
is to be used in the Bessemer converter. 

After the blow, and while the converter is turned down, a 
sufficient amount of high manganese pig is thrown in for the pur¬ 
pose of giving the metal the proper amount of carbon, and, at 
the same time by means of the manganese, secure a more homo¬ 
geneous metal, by deoxidizing the bath of any dissolved oxygen. 
It acts also as a desulphurizer, by forming a sulphide of mag- 
ganese, which is soluble in the slag. The metal is then poured 
into a ladle, and from this it is poured into the iron moulds. 
The characteristics of the process are rapidity of action ( 10-18 
minutes per blow), the melting of the metal in a receptacle 
other than the converter, the small charges handled ( 10-15 tons) 
and the heat imparted by the energetic combustion of the im- 

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purities. The above described process, which is common to 
America, is called the acid Bessemer process. Most operations 
are carried out in the large plants without recourse to manual 
labor. 

Germany is the principal country to operate the Bessemer 
method with a view to the elimination of phosphorous. They 
accomplish this by charging into the converter sufficient lime¬ 
stone to give the resultant slag its basic tendency, thus enabling 
it to dissolve the phosphorous and also considerable sulphur 
as well. For this reason they are able to use a higher phos¬ 
phorous iron in their work. It is necessary, in this process, 
to recarburize the metal after the same is poured from the con¬ 
verter, because the impurities and carbonic oxide formed, 
would tend to reduce the oxide of phosphorous that was in the 
slag, and thus rephosphorize the metal. 

The next great process is the open hearth process; and it 
may, like the Bessemer, be classed either as acid or basic in its 
operation. The acid process is the least used of the two, but 
turns out a product generally agreed to be the better. The 
open hearth furnaces are large enclosed hearths, which are lined 
with refractory materials, and supplied with ports for the gas 
and air, which substances enter in a heated state, and unite, 
bringing about a temperature high enough to melt the iron 
placed on the hearth. Air is supplied in excess, so as to secure 
the oxidation of the impurities in the bath which , as the same 
is shallow, and agitated by boiling, is constantly causing the 
impurities to come into contact with the ogygen of the air at the 
surface of the metal. This process is a long one and six to ten 
hours are required for purification. The process would take a 
longer time if the air alone were relied upon as the oxidizing 
agent, so iron ore, which is rich in ogygen, is charged to hasten 
the action. In the acid method the hearth is lined with an acid 
material, and the charge is pig iron and scrap, this latter put in to 
reduce the carbon percentage of the mass. As in the Bessemer 
converter, silicon and manganese are the first elements to be 
burned out and these form a fluid slag, which floats on top of 
the metal and protects it from the intense heat of the flame. 
This slag is acid in its nature and will not hold a large amount 
of phosphorous or sulphur, and therefore when steel is made 
by this method, a pig iron low in these elements is a necessity. 
Tests are made, as the process proceeds, and when the carbon 
is oxidized the flame is shut off. Then the recarburizer is 
charged and the metal run into the moulds. 

The basic open hearth process is the same as the acid, 
except that the lining is basic in character and a charge of lime¬ 
stone is used to make the slag so basic in quality, that it will 

8 


hold the oxidized phosphorous. The recarburizer is put in 
after the metal is poured, for the same reasons as stated under 
the basic Bessemer process. 

The chief characteristics of the open hearth process are the 
long time for purification (six to ten hours), and the large 
amount handled (thirty to seventy tons). It gives a better 
metal than the converter process, due probably to the long 
time taken and the large percentage of scrap used. 


THE CONSTITUTION OF STEEL 

A study of the constitution of steel is perhaps best pre¬ 
ceded by a short review of the nature of solutions, in order to 
better understand the changes that take place, as steel cools 
from its high temperature down to that of the atmosphere. 

When a liquid dissolves a substance we have a solution. 
The substance dissolved is called the solute, and the dissolving 
substance the solvent. Some substances will dissolve in all 
proportions; others only in certain proportions. In the latter 
case the solution is said to be a saturated one when the solvent 
is holding all the solute that it can. If more than this amount 
be added the solute will precipitate, since the solution is already 
saturated. As a rule an increase in the temperature will cause 
an increase in the dissolving power of the solvent; and hence 
at a higher temperature the precipitate will disappear in .solu¬ 
tion. Gold and silver are examples of substances that dis¬ 
solve in each other in all proportions, whether liquid or solid. 
When a liquid solution of either one in the other freezes or 
solidifies, both substances will freeze out together in the same 
crystal. When substances freeze in this manner the resulting 
solid mass is also a solution of the one element in the other, 
only in this case it is a solid solution and not a liquid one, since 
the crystals of one are dissolved in those of the other. 

Tin and lead are examples of two elements which when 
liquid may be in solution in any ratio, but since they cannot 
dissolve each other when solid, we have when the mass is frozen, 
not a solid solution, but an intimate mixture of crystals of lead 
and tin. The solidification, in this case, takes place as a sat¬ 
urated solution. The solution cools and reaches a temperature 
at which the solvent is saturated with the solute. Further 
cooling makes the mass supersaturated, and the substance in 
excess precipitates, while at each degree drop in the tempera¬ 
ture the precipitated solid mass increases, thus in turn enriching 
the liquid mass remaining, in its percentage of the solute. 
After a certain temperature is reached, this liquid remaining, 

* 9 


called the mother liquid, will have attained certain proportions 
of the solute; and further drop in the temperature will not 
cause any more freezing of the excess metal alone. Instead, 
at this point, the solution will solidify in intimately mixed crys¬ 
tals of lead and tin, and not in solid solution as occurred with 
the gold-silver alloy. The latent heat given up at this time is 
enough to seriously check the drop in temperature of the mass, 
and until this last alloy is frozen, the temperature is almost 
stationary. This last freezing alloy of elements in such pro¬ 
portion that both solidify together, is called the entectic, which 
means “ well melting ” solution, or the solution with lowest 
melting or freezing temperature. 

Now consider steel as composed of its two most abundant 
elements, namely iron and carbon, and we get on solidification 
(if the carbon does not exceed 2.2 per cent) a solid solution, 
or a solid formed in a manner similar to the alloys of the gold- 
silver series. In this case the two crystallizing substances are 
iron and probably cementite (often called iron carbide), since 
this is the form in which carbon seems to exist when in iron. It 
is a compound of one atom of carbon with three atoms of iron, 
thus taking the chemical symbol Fe 3 C, and it is very hard (H=6). 
There is some doubt as to whether when the mass is molten, 
the carbon exists in this form; but from what occurs later it 
would appear as if this was the condition when the mass had 
solidified. Therefore, we may again state, that the solid 
solution will consist of crystals of iron and crystals of cementite 
dissolved in each other; and this will occur for all proportions 
of carbon up to 2.2 per cent. These amounts of carbon cover 
all grades of steel; so we may say that all steel freezes in solid 
solution. This solid solution has been independently named 
anstenite, and as it cools a strange change takes place. We 
find that the solid solution does not survive to ordinary temper¬ 
ature, but when a certain temperature is reached, precipitation 
takes place according to the laws of the lead-tin series, the 
only difference being that in this case, they take place within 
the solid mass instead of from a liquid condition. If the steel 
be hypoeutectoid, that is have less than 0.9% carbon in solution, 
then the iron of the austenite will separate out first, because it 
will be in excess as certain temperatures are reached, and in 
this state it is practically pure, and is called ferrite. In other 
words the anstenite has become supersaturated with ferrite 
with decline in the temperature, and as it cannot remain in 
solution, it of necessity precipitates. This precipitation, as 
with the lead-tin alloys, causes an enrichment of the remaining 
anstenite in the other component cementite, so as to soon bring 
about a 0.9% carbon ratio of the remaining solid solution. 

10 


Beyond this the ratio cannot increase, and the remaining 
austenite is now precipitated into narrow layers of ferrite 
mingled with layers of cementite. This final conglomerate is 
called pearlite, from its “ mother of pearl ” appearance, and 
the layers are not much more than 40 ,Soo of an inch in 
width, so that it takes microscopes of high power to bring it out. 
This substance, pearlite, or solid solution alloy of lowest trans¬ 
formation point, is called the eutectoid; and it is a conglomerate 
formed within the solid metal in the same manner as the eutectic 
is formed from the liquid solution. Likewise had the steel 
been of just 0.9% carbon composition to start with 
there would have been no excess substance at all, and the entire 
mass of austenite would be transformed into pearlite when the 
transforming point was reached. The temperature for this 
eutectoid formation is about 1290°F.,‘and so from the above 
discussion we see that all steel cooled slowly past this point 
must finally consist of either ferrite or cementite mixed with 
pearlite, or of pearlite alone. The ratio of cementite to ferrite 
in this eutectoid pearlite is approximately 1 to 6.4. The 
pearlite can be calculated for any steel as follows:— 

WHEN CARBON IS 0.9% OR LESS 

%CX15=A=% cementite which exists in the pearlite 
A X 6.4 = B = % ferrite that exists in the pearlite 
A-j-B=C = %of pearlite in the mass 
100 — C = Excess of ferrite in the mass. 


WHEN CARBON IS OVER 0.9% 

%CX15 = A= % cementite in the mass. 

100 — A = B —% ferrite in the mass. 

- = C = % cementite in form of pearlite. 

6*4 ... 

A_c = D = Excess of cementite in the mass. 

100 — D = % of pearlite in the mass. 

The ferrite formed in slow cooling steels is very pure iron; 
ductile magnetic, of about 45,000 lbs. tensile strength and is 
called alpha ferrite. This is to distinguish it from the forms in 
which it exists when above the saturation temperatures. Above 
these points the ferrite is chemically the same as below, but 
has assumed a different allotropic condition, so that when in 
the solid solution state of austenite it is known as gamma ferrite, 
and it is much harder than the alpha ferrite, and non-magnetic, 
besides. Likewise between these two forms there comes into ex¬ 
istence a third, called beta ferrite, which is very hard and but 


11 



slightly magnetic. In low carbon steels, say under 0.3%, it is pos¬ 
sible to distinguish these three states or allotropic changes in the 
ferrite, but as carbon increases beyond this point they run 
closer together, until the points of transformation appear as 
one. It is customary to call these points of transformation 
the critical points, and when they occur in the process of cooling, 
they are further called the points of recalescence. It should 
again be remembered, that at these points no chemical changes 
in the ferrite occur, but only allotropic modifications, and that 
these take place in the ferrite whether it is combined with the 
carbon forming cementite, or whether it is separate as excess 
ferrite. The cementite, when composed of gamma or beta 
ferrite, is much harder than when composed of alpha ferrite, 
and it is less magnetic. From the above I think it can be now 
seen, that as the solid solution austenite (cementite of gamma 
ferrite and a separate mass of gamma ferrite dissolved in each 
other) cools, there must be at different stages through the trans¬ 
formation range masses of crystals more or less fully developed 
towards the final structure, which would be that of the eutectoid 
pearlite. 

These various conditions have been named, and though 
their compositions are not fully known yet many of their physical 
characteristics are; and it is now agreed that they occur in the 
following succession, namely: austenite, martensite, troostite, 
osmondite, sorbite, and pearlite. Now as the critical points 
of steel are reversible upon reheating, that is to say, since we 
can by heating a cold piece of stock up to about these points, 
obliterate the pearlite or normal structure and cause the cementite 
and ferrite to pass from alpha form back into solution in the 
gamma form, it is evident that by quenching steel from different 
points within the transformation range we should be able to 
retain some one of the above forms in the cold. The most prac¬ 
tical method, however, is to cool from above the last critical 
point and obtain the different forms either by altering the rate 
of cooling or by subsequent tempering. It is evident, now, that 
the explanation of the hardening of steel by quenching is in the 
retention of some one of these hard forms, which in turn is due 
to the fact, that a certain amount of time is required if each of 
the transitions is to take place, and the quick cooling due to 
quenching prevents or obstructs the change. For example, 
assume the steel quenched. Then as the temperature passes 
the critical point in cooling, the desire of the structure to reach 
its normal state steadily increases; but its attainment is offset 
by the steadily increasing molecular resistance, due to the chilling 
of the plastic mass. On the other hand, the explanation of 
softening by annealing, or slow cooling, is that plenty of time has 

12 


been allowed for complete change from the harder forms 
of gamma or beta ferrite to the softest variety, namely alpha 
ferrite. It may thus be readily seen, from what has gone 
before, that the hardness and brittleness of steel increase with 
the rapidity of cooling below the critical range. The idea that 
steel can be made harder by heating it to temperatures well 
above the critical range is a fallacy, because once the cementite 
and ferrite are transformed from their normal state into the 
solid solution form of austenite, further application of heat is 
useless. The reason for the higher hardening power of high 
over low carbon steels is due, not to the carbon itself, but to 
the form in which the iron or ferrite exists. It seems as if 
carbon brought about a brake action effect upon the tendency 
to change from austenite to pearlite; and the higher the carbon 
the more the brake power exerted and consequently the harder 
the steel. Increased carbon means more cementite in the mass, 
and though a quantity of this substance in itself may make 
steel hard, yet it exerts, because of its carbon, a greater effect, 
by promoting the brake action on the change of the austenite 
into the more normal forms of ferrite and cementite. 

Since the bulk of hardening steels consist in their normal 
state mostly of pearlite, it becomes of great importance to be 
sure of our determination of the points of recalescence, and to 
assure ourself in regard to the reversibility on reheating. As 
before stated, a temperature of about 1290°F seems to be the 
point of commencement of change of state from austenite into 
pearlite. Probably this temperature is a little below the true 
point of transformation, because of the natural inertia to change 
of form, and perhaps because of some phenomena akin to 
surfusion. At all events, it is found that in practice, when we 
heat steel before quenching, it is necessary to go to a tempera¬ 
ture some 80° F to 100° F higher; this difference probably 
being due in its turn, to the natural inertia to change from the 
form of pearlite into that of austenite. Since these points on 
reheating are different from those of cooling they go under 
the name of the points of decalescence. It is a fact, however, 
that we could bring the change about by heating to 1290° F 
or slightly higher, but only by allowing the steel to stand at 
this temperature for a considerable number of minutes. To 
hasten the change it is necessary to heat, as stated above, 
to a temperature 80° or 100° F higher, which would mean a 
working temperature of about 1370° F to 1390° F. In practice, 
temperatures of from 1400° F to 1600° F are often used, but 
as previously stated, when once the transformation has taken 
place further rise in temperature is useless, and besides only 
tends to deteriorate the structure, as will later be shown, by 

13 


making it coarser; and there are good grounds for believing 
that a structure like this makes the steel less difficult to pene¬ 
trate, or in other words, makes it less hard. All grades of steel 
will harden to some extent, though not until we get above .3% C 
composition will it be very noticeable. A 0.4% carbon steel 
will harden so as to cut soft iron and hold an edge, but most 
tools of ordinary carbon steel run 1.0% carbon and more. 
Sudden cooling to below 200° F is necessary if the steel is to 
be very hard. Any check in the rate of cooling will result in 
temper drawing, and hence change to one of the lower transition 
structures. This is the reason for a less hard steel when oil 
is used as the quenching medium. It is equivalent to hardening 
in water, and drawing the temper. An equal degree of hardness 
will best be formed by an even and uniform heat, and it is nec¬ 
essary, if we wish to escape strains of an uneven character, and 
consequent shrinkage cracks. A bath of even temperature, 
high specific heat, viscosity, and conductivity is essential, if 
we would have the greatest chilling effect. Agitation of the 
quenching mediun is also effective, in that it serves to brush 
off particles of steam or gas that may collect on the parts, 
such gases being detrimental, inasmuch as they are poor heat 
conductors. Hardening increases the tensile strength and the 
elastic limit of the steel; but greatly reduces its ductility and 
ability to withstand sudden shock. In order to partly restore 
this latter characteristic, tempering is resorted to which consists 
of a reheating of the hardened product to temperatures, as a 
rule, between 400° F and 600° F. The theory of tempering 
is that hardened steel at atmospheric temperature is under 
a natural strain, the tendency of which is to cause the structure 
to revert to its normal condition as soon as conditions are made 
under which it may exert this desire. A slight reheating favors 
the change and allows of a transformation of some of the mole¬ 
cules to their normal condition, thus increasing the ductility 
of the mass, and at the same time decreasing the hardness. 
After heating to temper, it is immaterial whether we cool 
quickly or slowly since the degree of temper is not altered. 

With ordinary carbon steels it is difficult to harden the 
metal so that austenite will be retained in the cold. In fact we 
cannot obtain it pure, since there is always time, no matter 
how sudden the chilling, for partial transition to take place. 
Some may be obtained if a steel high in carbon is quenched from 
a high heat; but it is very unstable at ordinary temperature 
and is readily changed to the next form, martensite, upon appli¬ 
cation of heat. As before stated it is the solid solution of 
cementite in gamma ferrite. Above the critical range all 
steels are in this form, and only at temperatures above this is 

14 


it stable. Elements such as manganese and nickel seem to 
aid the carbon in retaining steel in this form. 

Martensite is the next transition form and it is the usual 
constituent of hardened carbon steels. It is a form more stable 
than austenite and considerably harder, probably consisting 
of a mixture of cementite in gamma, beta and alpha iron; 
obtaining without doubt, its magnetic properties from the 
alpha iron in the mass. The application of heat changes it 
to the next forms, first of which is troostite; and so we find 
this form in all tempered steels. Troostite is formed by quench¬ 
ing from within the critical range of temperatures, or by quenching 
from above that range in hot water. It is found in the central 
parts of large hardened pieces, due to the slow cooling actions 
inside of them. It is softer than martensite, but probably 
like it consists of cementite in gamma, beta, and alpha ferrite* 
but with a much greater proportion of alpha ferrite. 

Osmondite is the next form and one but recently discovered. 
It is principally a solution of cementite in alpha ferrite. 

After osmondite the next form to appear is sorbite. It 
seems to be a form very close to pearlite, but with crystals 
of cementite and ferrite imperfectly formed. It can be ob¬ 
tained by quenching from temperatures near the lower end of 
the critical range, or by chilling in oil. A temper of about 
600° F seems also to form it. It makes a very tough structure 
and is softer than any of the above forms. 

Pearlite, the normal and next form, is, as previously stated, 
a mixture of layers or crystals of cementite and alpha ferrite. 
Its composition is 0.9% carbon and it is formed in all steels 
by slow cooling, being either alone as in a steel of 0.9% carbon, 
or mixed with an excess of ferrite or cementite depending upon 
whether the metal was hypo- or hyper-eutectoid in its compo¬ 
sition, that is as to whether it contained less or more than a 
0.9% carbon. The tensile strength of steel is greatest when 
about of this proportion. 

Alpha ferrite, as found in annealed steel, is almost pure 
iron, and is very magnetic. It carries in solution and com¬ 
bination small amounts of the elements silicon, manganese 
and phosphorous that are found in steel. It is soft, very ductile 
and is the chief constituent of low carbon steel. Wrought 
iron is the nearest commercial product like it. Very slow 
cooling through the critical range is necessary for its complete 
formation; though a quick and quite satisfactory method is to 
heat to a point just below the range and then suddenly cool 
by quenching. This process is called water annealing. 

The elements silicon, manganese, sulphur, phosphorous, 
oxygen and possibly nitrogen are present in all steels to a more 

15 


or less degree. The first combines with iron to form possibly 
a series of silicides, most important of which is the one of symbol 
FeSi. Silicon up to a percentage of 0.6 is not considered, as 
a rule, to be harmful in its effect on soft steels. Silicon is 
sometimes used as a deoxidizer because of its great affinity for 
free oxygen; but it usually gets into the metal from the pig 
iron used for making the steel. 

Manganese, like silicon, is a great purifier, in that it deox¬ 
idizes the bath. It is for this reason that a recarburizer high 
in this element is used. If its percentage does not exceed 1.0 
it is generally believed to confer no bad properties upon soft 
metals; in fact it increases the tensile strength of steel as it 
increases beyond 0.4%. It seems first to combine with all the 
sulphur it can take up forming the compound manganese sul¬ 
phide ( Mn S). This sulphide occurs in drops, and upon rolling, 
the tendency is for the.se to spread out into thread form, or 
“ ghost lines,” as they are called in the mills. It weakens 
the steel; but, though a brittle form, it is very advantageous 
to have as much as possible in this condition; and hence 
there should be manganese enough to secure all the sulphur. 
Though a little less than twice as much would be the theoretical 
amount, yet it is found that when its amount is at least four 
times the sulphur content the combination is easier effected. 
Part of the manganese combines with carbon to form a hard 
carbide ( Mn 3 C) which is often found mixed with cementite, 
both being at times, in the same crystal. Manganese tends to 
make the grain finer, thus offsetting the coarsening action of 
phosphorous. Manganese seems to have a tendency to cause 
steel to crack upon quenching. 

Sulphur, as stated above, will all combine with manganese 
if there is sufficient of the latter present. What does not occur 
in this form will combine with iron to form iron sulphide ( Fe S). 
In this state it will make the steel more brittle than in the form 
of manganese sulphide, for whereas manganese sulphide dis¬ 
tributes itself throughout the mass in the form of drops, the iron 
sulphide, on the other hand, will be found developed in sheet 
form, which naturally is more weakening to the structure since 
it prevents close adhesion of the crystals. Besides, at rolling 
or forging temperatures, both forms of sulphide are fluid, and 
this is injurious, inasmuch as it causes cracks to develop. This 
explains the so-called “ red shortness ” of steel of high sulphur 
content. 

Most injurious of all elements known is phosphorous, 
since it makes steel brittle and renders it unable to withstand 
shock or hard service. It dissolves in the iron, and as solidifi¬ 
cation of the steel proceeds, it separates as a eutectic of such 

16 


a low freezing temperature, that it is fluid after the great bulk 
of the metal has solidified. This allows the eutectic to flow 
around the formed crystals and thus make a brittle network. 
Its effect is less severe in low carbon steel, because with ferrite 
in such excess there is less of the entectic formed. For these 
reasons it is always well to keep the content of phosphorous 
low in all steels, especially those of high carbon content. Its 
tendency is to make the structure of the steel more coarse. 

Oxygen forms oxides which are harmful to either soft or 
hard steels, these compounds acting much the same as sulphur. 
Bessemer steels are apt to contain more of these compounds 
than those made by other methods. 

The more evenly distributed are the above impurities the 
better will be the finished steel, but as is well known to the steel 
maker, they tend to collect or segregate. The cause of this 
action is that when the steel, which has been poured into the 
moulds, commences to solidify, these elements, or their com¬ 
pounds, which at the high temperature of the molten metal 
are in solution, are in turn rejected from the freezing crystals 
of austenite. In other words the crystals become supersatur¬ 
ated with them upon freezing, and throw off the excess which 
are at once immediately redissolved in the remaining hot liquid 
mass within the outer frozen walls, thus at the same time enrich¬ 
ing it in these impurities. This process continues until that 
part of the ingot to last freeze, will, as a rule, be found richest 
in these elements, and this point is usually in the inner upper 
third of the ingot. At this same point is formed a pipe or 
cavity due to the continual shrinkage away from the last 
frozen mass of the exterior walls, which are freezing ahead of 
it. The pipe may be lessened by slow pouring, or by stirring 
of the top of the ingot so the same cannot freeze over. Segre¬ 
gation is lessened by the use of small ingots, which promote 
quick cooling of the mass. It is usual for this top third of the 
ingot to be sheared off and scraped so that this segregated 
portion can be kept out of the finished product. Aluminum 
seems to offset segregation by causing less agitation from 
escaping gases; for if agitation occurs the crystals that are 
forming out into the molten interior or open sea, as it is some¬ 
times called, are not so apt to embrace the impure elements 
that are being rejected, and hence they become redissolved 
in the molten interior. Gases in solution, as well as the other 
elements noted, are also rejected as the crystals freeze; and 
if they get caught in the crystals the result is a blow hole in 
the steel, which may be minute or comparatively large. If 
the blow holes are well inside, the mechanical work done on the 
ingot will close them up, and the resultant bad effects, which 

17 


otherwise would appear, will be well eliminated; but if they 
are near the edge their walls become oxidized on exposure to 
the air, and they then do not easily weld together. The blow 
holes seem to be deep seated as the percentage of manganese 
plus 5.2 times the percentage of silicon decreases from 1.66. 
One possible cause of the gases is the reducing action of carbon 
in the recarburizer upon the oxides of iron which are always 
present, the resultant gas being carbonic oxide. The use of 
elements such as silicon, manganese, aluminum, vanadium, etc., 
are often resorted to in order to purify the metal of the dis¬ 
solved gases. 

We will now consider heat treatment as it effects the size 
of the grain, and reference will be made to the freezing curves 
of steel. Chart I is that part of the Roberts-Austin diagram 
which pertains to steel, it being carried further to the right in 
the original to cover irons of higher carbon percentage, namely 
cast irons. It ends here, at the two per cent point, which is 
about the steel limit. The line A o gives the temperatures 
at the commencement of freezing of the molten steel and the 
line A a gives the temperatures at the end of this action. From 
here down to the line G H S E the steel is in the condition of 
austenite, or a solid solution of cementite in gamma ferrite. 
G H S represents the temperatures for steel under 0.9% carbon, 
at which the excess substance, ferrite, commences to precipitate, 
as before explained, changing first into beta and finally into 
alpha ferrite. The chart shows that to the left of point H, 
which represents a carbon steel of about 0.3%, the critical 
points designating change of form from gamma into beta and 
then alpha iron are decernable; but to the right of this point the 
upper critical point has so run into the second that they appear 
as one in the line H S E. The third critical point is that of 
separation of the eutectoid, namely the temperature as shown 
by point S. 

The figure shows also that while the temperatures for the 
formation of ferrite ( G H S) or cementite ( S E) vary with 
different grades of steel, yet for all of them the temperature 
at which the eutectoid forms in the same, namely S. 

Region III, on the chart, is that of pearlite and alpha 
ferrite, this domain being to the left of the 0.9% carbon point; 
while region IV is that of pearlite and cementite, this domain 
being to the right of the above mentioned point. A steel of 
exactly 0.9% carbon, consists of pearlite alone, when in the nor¬ 
mal state. It should be stated that since the chart represents 
the freezing points, it is necessary on reheating to heat to points 
about 80° F to 100° F higher in order to reverse the processes, 
this being caused by molecular inertia, as previously explained. 

18 


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19 






































At the temperature of point S we get the finest grain in 
most steels. At each degree rise in temperature above this 
point the grain size will increase almost exactly in proportion 
to the increase of temperature. This sadly weakens the steel, 
especially those of high carbon, both strength and ductility 
decreasing; but in low carbon stock this latter quality is not 
altered so much. To whatever temperature above S the steel 
is heated we will bring down from that point the grain size of 
that temperature, and it can only be gotten rid of by special 
heat or mechanical treatment. This increase in grain size is 
often not noticeable to the eye, but nevertheless it is there if 
we have heated our stock above the point S. In order to 
eliminate this grain size, we can reheat the stock again to 
temperature S, and all the pearlite will go into solid solution 
in the finest grain form. Heating to any point short of this is 
ineffective, and our original grain size is not altered; while 
heating beyond point S immediately results in the steady en¬ 
largement of the newly formed fine grain of the solid solution 
which forms at this point. We should reheat low carbon 
steels to a point higher than temperature S, because, though 
as stated above, this is the point for a new crystallization of 
the pearlite which is forced into solid solution, yet, on the other 
hand, there will be a considerable quantity of free ferrite, which 
substance will not attain its finest grain until it in its turn is 
forced into the solid solution. The temperatures for this change 
are those at which it separated out as the excess substance on 
freezing, or the temperatures of the line G H S. In other 
words only the eutectoid will go into solution at the point S; 
and we must heat higher to return the excess substance, ferrite. 
Of course, in heating above the point S, the newly formed fine 
grain of the solid solution will at once commence to increase, 
so that while we are trying to refine the ferrite by further heating, 
on the other hand we are destroying the newly crystallized 
grain size of the solid solution; but since the ferrite is so large 
in amount (in steel under 0.5 % carbon it is in excess), it is 
deemed advisable to follow this method. The above explana¬ 
tion is an answer as to why a steel of this composition cannot be 
finely refined. 

Theoretically we ought to secure the metal of best grain 
by heating to S a steel of the entectoid ratio, namely 0.9% 
carbon, because when this temperature is reached the entire 
mass will go into solid solution, and further heating will be un¬ 
necessary. This is, perhaps, a cause of the great strength 
shown by eutectoid steel when properly heat treated. 

With steels over 0.9% carbon it would be necessary to 
reheat to the line S E if we were to force into solid solution the 

20 


excess substance of cementite and thus refine that portion, 
but since in steels of this composition the pearlite is so much the 
greater in amount, it is better to reheat only to temperature S, 
and thus not cause a re-enlargement of this finely formed grain, 
for the sake of refining the small excess of cementite that 
might be in the mass. In hypo-eutectoid steels these grains 
are surrounded by ferrite, and in hyper-eutectoid steels by 
the excess substance cementite. The number of grains to the 
square inch in well refined steels of fair carbon percentage may 
be as much as ten thousand or more. 

The other method of refining the grain, though not so 
effective as the heat treatment method and not always appli¬ 
cable, is to break down the grain size by mechanical work. 
This is the only method known which can be used in the making 
of large pieces, and it is for this reason that great care is taken 
in the rolling and forging of the same. A high temperature 
is necessary if the ingot is to make many passes through the 
rolls, and certainly one higher than that of finest crystallization, 
namely S. Consequently when the rolling commences, we have 
in the mass a coarse crystallization. Constant work upon the 
part will break down this grain; and if we continue rolling or 
forging until temperature S is reached, we will have attained 
the finest grain size that can be got by mechanical working. 
If we stop at any point above S, the grain size of that tempera¬ 
ture at once forms and will come down in the part if no more 
work is done. Therefore mechanical work should be continued 
until the line G H S P' is reached; but when carried beyond 
these temperatures strains will be set up in the part. All large 
pieces show different grain size because the centre must be 
always hotter than the surface layers; and for this reason it is 
considered good practice in the mills, to roll the shapes at the 
proper temperature when the greatest amount of work is being 
done. A welded piece is often stronger in the weld because it 
is at this place where the greatest amount of work and con¬ 
sequent reduction of grain has occurred; while in sections upon 
either side the coarse grain of the high welding temperature 
is to be found. Hence it is weaker here and more apt to break 
at these points. Therefore all welded parts should be given 
heat treatment, as above described. 

In practice we often heat treat by heating to the line 
G H S P' and then after quenching draw the temper. Quench¬ 
ing from these points refine and harden the stock, and it is to 
relieve it of all hardening strains that the temper is drawn to 
high points. This changes all austenite into the lower forms 
of sorbite or pearlite, and we consequently obtain the strongest 
metal consistent with good ductility. 

21 


It can now safely be stated that crystallization of steel 
through service cannot take place. Heat is the necessary 
adjunct for crystallization. The common belief that crystal¬ 
lization will occur through continuous work is a fallacy. To 
be sure we see in certain fractures a coarse grain, but all evidence 
tends to prove that it must have been there in the beginning. 
There are, in fact, good grounds for believing that constant 
straining of large crystals would, in time, pull them out into 
a fibrous form. 

In annealing we should heat to the line G H S E to secure 
complete transformation into austentite and then cool very 
slowly in order to transform this substance into pearlite and its 
excess member; but if we wish to attain the best grain size with 
softest condition, we should double anneal the stock by first 
heating to the line, as stated above, and then give the part a 
second reheating to the line G HSP' to obtain the best grain. 
Besides its softening effect, annealing is a factor in removing 
strains caused by uneven cooling among the different crystals. 

Burnt steel cannot be refined; but burning takes place 
only at temperatures much higher than is ordinarily used in 
practice. The crystals here seem to be forced apart, which 
possibly may be caused by the formation of gases either by 
chemical action or by their being thrown out of solution by the 
high temperature. Mechanical work may, to some extent, 
better such steel by closing up the grains, but for most purposes 
such stock is worthless. 

The cuts on plate II may now be of assistance in explaining 
this discussion of grain size and structure. 

Figure 1 may be assumed to represent the magnified condi¬ 
tion of a piece of stock heated to a temperature well up in region 
I. The network defines the grain size and conveys to us the 
idea of the space that exists between the crystals. The condi¬ 
tion of the grain or crystals is that of austenite, or the solid 
solution of cementite and gamma ferrite. 

Figure 2 represents the above piece of steel after the tem¬ 
perature has slowly dropped down to that of region II. The 
increased heaviness of the network indicates that the spaces 
between the crystals have been somewhat filled with ferrite, 
(if carbon is less than 0.9%), or cementite (if carbon is more 
than 0.9%) which must precipitate from the austenite as soon 
as temperatures of the line G H S E are attained. 

The iron or ferrite at this time is of alpha modification, it 
having probably passed through the beta stage in reaching this 
condition. The grains themselves, are still austeinte in char¬ 
acter. 


22 


PLATE II 




2 5 8 



23 






Figure 3 represents conditions after the temperature has 
past the line PSP'. The network has now attained its maximum 
thickness of ferrite, or cementite, as the case may be, and the 
constitution of the enclosed grains is pearlite, the transforma¬ 
tion from austenite having occurred as the temperature passed 
the point S. 

Figure 4 shows the structure after this same piece is reheated 
to the line G H S E and subsequently quenched. Let us state 
that when temperature S was reached, the coarse grains of 
pearlite werfe suddenly changed into austenite in the finest 
grain size. In other words the steel had been refined. Further 
heating across the critical range resulted in a gradual weakening 
of the old network, as the substance of which it was composed 
became redissolved by the austenite meshes. While this orig¬ 
inal coarse network was being obliterated, the newly formed 
fine grain within it was gradually becoming more coarse, because 
the temperature was rising above that of point S, the 
point of finest grain formation. Therefore our figure shows 
a refined structure with the old grain obliterated, yet a structure 
somewhat coarser than it otherwise would have been had we 
not sought to rid it of the above mentioned network. The 
constitution of the mass would be entirely austenite to the solid 
solution of cementite in gamma ferrite if the cooling were 
instantaneous, but because this is not the case the form assumed 
will be one lying between that of austemite and pearlite, this 
depending upon a variety of conditions, as was previously shown. 

Figure 5 shows the structure of this same piece after it has 
been annealed from the line G FI S E. Note the network of 
the excess substance due to prolonged cooling through the critical 
range. Note also the refined grain and the entire obliteration 
of the old grain size. The constitution of the network may be 
either ferrite or cementite, and the grains are pearlite. 

Figure 6 represents a steel of eutectoid proportions, namely 
0.9% carbon, after heating to a temperature high in region I. 
The grain size is large as was the case with Figure 1. The 
constitution is in the form of austenite. 

Figure 7 shows the same piece of 0.9% carbon steel after it 
has slowly cooled below the temperature of point S. Note 
that here there is no precipitation into the network since neither 
ferrite or cementite are in excess. The constitution of the 
mass is entirely pearlite. Note that the grain size of the high 
temperature is retained in this piece. 

Figure 8 represents the same piece reheated to the point S 
and subsequently quenched. Note the refinement of the grain 
which should be the best of any steel since stock of this compo¬ 
sition all goes into solid solution when temperature S is reached. 

24 


The constitution will be of a form lying between austenite and 
pearlite. 

Figure 9 shows the result when this same piece is reheated 
to line PSP' and very slowly cooled or annealed. Note the 
refined grain as compared to that of Figure 7. The constitution 
of the mass is that of pearlite entirely, there being no substance 
in excess to precipitate upon cooling. 

Regarding the physical tests to which steel is usually put 
we may mention those of ultimate tensile strength, elastic 
limit for tension, ductility, and the tests for resistance to “ fa¬ 
tigue.” Less frequently tests are made for compressive strength, 
torsional strength and degree of hardness. These properties 
will vary greatly depending, as before stated, upon the condition 
of the structure, chemical constitution and kind of mechanical 
treatment. With ordinary steel whatever tends to increase 
the ultimate strength decreases the ductility or toughness. 
Thus, hardening, and mechanical working increase the ultimate 
strength and reduce the ductility, whereas annealing increases 
the ductility but decreases the strength. It is impossible to 
get maximum strength and ductility at the same time. 

Carbon is the great strengthener of ordinary carbon steel, 
and more strength is gained by its use with less loss of ductility 
than by any of the other elements. The strength increases 
in proportion to increase in carbon until about 1% is reached, 
after which there is a decrease. Thus if one specifies high 
strength they should not set a limit on the amount of carbon, 
since it is the best ingredient by means of which high strength 
can be attained. 

If silicon does not exceed 0.6% in soft steel its effect on 
the strength is slight, and no bad qualities are conferred on the 
metal. 

Manganese adds to the strength after amounts of 0.3 to 
0.4% are attained, and its effect increases as the carbon increases. 
Enough should be present to counteract the “ hot-shortness ” 
or liability of cracking when rolled that the element sulphur 
seems to cause. Manganese and silicon are kept lower in tool 
steels, say 0.4 and 0.2% respectively. 

Sulphur does not appreciably effect the ultimate strength 
or other properties of cold steel. The amount should be always 
low to avoid “ hot-shortness.” 

Phosphorous increases the ultimate strength, but because 
of its deteriorating influence thanks to its making the metal 
“ cold-short ” or brittle, it should always be as low as possible. 
In tool steels this element and sulphur should be very low, say 
0.02 and 0.04% respectively. 

After a study of the records of hundreds of tests on acid 

25 


and basic open hearth steel, Campbell evolved a set of formulae 
for the ultimate tensile strength of this kind of steel. He 
checked many future heats with them, and its accuracy was 
proven within fair limits. The influence of silicon and sulphur 
is neglected, and only that of carbon, manganese and phos¬ 
phorous taken into account. The variable R is added to cover 
variations in strength due to different conditions other than 
those of a chemical nature. R becomes a + quantity where the 
part is small in section or the temperature conditions low during 
rolling. If the temperature conditions are high during rolling 
or if the part in large in section R becomes a — quantity. 
The formulae follow: 


Acid Steel Ult. tensile strength = 40000 +1000 C +1000 P + X Mn + R 
Basic Steel “ “ “ =41500+ 770 C +1000 P + Y Mn+R 


The unit for carbon is 0.01 
“ “ phosphorous is 0.01 

“ “ manganese “ 0.01 

“ values for X and Y follow: 


PER CENT CARBON ACID STEEL 

X 

0.05. 

0.10. 80 

0.15. 120 

0.20. 160 

0.25 . 200 

0.30 . 240 

0.35 . 280 

0.40 . 320 

0.45 . 360 

0.50 . 400 

0.55 . 440 

0.60 . 480 


BASIC STEEL 
Y 

. 110 

. 130 

. 150 

. 170 

. 190 

. 210 

. 230 

. 250 


The values X begin when manganese content = 0.04 
Y “ “ “ 11 = 0.03 


i ( 


i l 


Thus if an acid steel analysis was as follows: 

C = 0.16 Si = .033 Mn. =0.40 S = 0.45 P = .060 
Then the tensile strength would be 

40000 + ( 1000) ( 16)+( 1000) ( 6)+( 120) ( 40) + R = 
65800 + R lbs. in which case R may be + or — as mentioned 
above. 

The ultimate strength as derived from physical tests of 
standard specimens is, of course, not used in the calculations 
of structures, until the amount has been divided by a proper 
factor of safety, to allow for the effect of variation in shape of 
the part and other inconsistancies due to the different local 
conditions that are bound to exist in each separate case. 


26 























The elastic limit is about two-thirds the value of the ulti¬ 
mate strength if the steel be mild. As the steel becomes harder 
this ratio is less. 

The elongation in mild steel will vary between 25 and 30% 
in a length of 8 ". As steel becomes harder this is greatly 
lessened. 

When a structure is subjected to repeated or alternate stress, 
the breaking load is much under the ultimate tensile strength 
of the material. 

Factors of safety for static loads run from three to four, 
and for live loads from six to eight. 

Turner states that since the elastic limit, reduction of area 
and dynamic alternations that a specimen will test out to 
represents the useful strength, static ductility and “ fatigue” 
resisting properties respectively, it is possible to embody 
all three in an expression such that when these three character¬ 
istics of the steel are known there will be a proper basis on 
which to make comparisons. He divides the product of the 
elastic limit, reduction of area and dynamic alternations by 
1,000,000 to obtain what he calls the “ quality figure.” Thus 
if E represents the elastic limit, R the reduction of area and A 
the dynamic alternations, then the “ quality figure F is ex¬ 
pressed as follows: 

E X R X A 

b 1 , 000,000 


ALLOY STEEL 

The desire within recent years to increase the ultimate 
strength, elastic limit and hardness of steel without destroying 
its ductility or toughness, has lead to the development of a class 
of steels popularly called alloy steels. It was found by inducing 
certain elements, either singly or together, in various propor¬ 
tions into steel of ordinary constitution, that many remarkable 
qualities were attained. The final results cannot always be 
predicted, since the addition of a few per cent more of an element 
or the mixture with it of one or more other elements will often 
absolutely reverse what seems to be already an assured property. 
These great changes are brought about doubtlessly by either 
the formation of peculiar and intricate carbides, or by the 
shifting of the critical points. As a rule these special elements 
are added to the charge like the recarburizer, and the principal 
ones in use to-day are nickel, manganese, chromium, silicon, 
tungsten, molybendum and vanadium. 

27 



Of the above substances the one most used is nickel. It 
is added, as a rule, to steel of medium carbon percentage, say 
from 0.2 to 0.5. The usual quantity added to steel that finally 
is to be subjected to stress, is about 3.00 to 4.50 per cent. It 
seems to make the steel more homogeneous than it otherwise 
would be, and tends to make it fibrous in structure. These 
qualities endear the steel with greater ultimate, elastic and shear¬ 
ing strength than would be realized with an ordinary steel of 
the same composition; yet at the same time its ductility is but 
slightly diminished. These properties, therefore, make it more 
valuable than ordinary steel when “ fatigue,” ( repeated or 
alternating stress), is the dominant factor. Though harder 
than ordinary steel it is, however, readily machined. An amount 
of 3.50% reduces the critical point ( on cooling) by about 170° 
F and this becomes still more as nickel is added. Thus instead 
of the change of iron from gamma to alpha form being complete 
at 1290° F, as should be the case with ordinary steel, it now 
does not complete the transformation until a temp, of 1120° F 
is attained. If the nickel is increased to 25% the change does 
not take place except at a temperature below that of the atmos¬ 
phere. In other words a steel of 25% nickel would be retained 
in its original state of solid solution after it had cooled; and 
obviously it would be non-magnetic. A still more peculiar char¬ 
acteristic of steels with less than 25% nickel is that this point 
of transformation for cooling is not reversible on reheating; 
that is to say, the metal is not turned back into solid solution 
at the same temperature. For example, a 20% nickel steel will 
be decomposed in cooling, at about 212° F, but it will take on 
reheating a temperature of about 1150° F to turn it back again 
into solid solution. The nickel of these steels seems to be dis¬ 
solved in the ferrite and cementite, the greatest percentage 
being found in the ferrite. Nickel steels of low carbon compo¬ 
sition do not take on a material hardness, when quenched, 
and hence if a hard surface is desired the stock is usually pack 
hardened, giving us, in this case, the hard surface sought after 
with a tough interior as well. 

Manganese when over 1% makes the metal somewhat 
brittle, and between 4 and 5% it is extremely fragile; yet when 
increased beyond 7% it results in a hard and ductile material, 
while with 12-15% the maximum strength and ductility are 
attained. Water quenching of this material increases these 
properties in which respect the results are the opposite of what 
would occur with like treatment to a carbon steel. This alloy 
is extremely hard and it cannot be annealed, so when parts of 
this material are wanted they are usually cast, and grinding 
takes the place of machining. It is not favored with a high 

28 


elastic limit. Its extreme hardness is due in part to the forms 
in which the manganese is combined, in the steel, but more 
particularly to the austenite state of the metal, a condition 
brought about because the transformation point on cooling is 
below atmospheric temperature, thus leaving the steel in this 
primarily hard condition of solid solution. Little is known 
about the reversibility of the critical points. 

Chromium acts as a hardener of steel because of the retard¬ 
ing action that it seems to produce upon the critical points for 
cooling. When quenched the steels are therefore quite hard. 
In proportions of 1.0 to 2.0% the resulting elastic limits are 
high. It is often used in combination with nickel. 

Silicon tends to aid carbon in its hardening action, and also 
makes for a fibrous material. Steels of very great magnetic 
properties are made by proper addition of this element. 

Tungsten and molybendum are the chief elements in the 
manufacture of the modern high speed steels, one per cent of 
the latter being as effective as two per cent of the former in pro¬ 
ducing the peculiar properties common to these steels, whose 
most important characteristic is that of “ red hardness ” or the 
ability of the stock to remain hard even though heated to red¬ 
ness. In other words a red heat does not decompose the solid 
solution. In these steels the critical range for cooling begins 
at about 1300° F and spreads out down to a temperature of 
about 600° F. By heating these steels in the neighborhood of 
2000° F and cooling moderately fast, the critical change is 
prevented and the metal is retained in the condition of austenite. 

Vanadium is an element but recently employed in the 
manufacture of alloy steel. Small amounts only seem to be 
necessary to confer important and peculiar properties upon the 
metal. It probably forms complex carbides, which in turn 
confer upon the steel a high elastic strength and at the same 
time great toughness. This element evidently serves as a 
scavenger for the oxygen and prevents segregation because of 
this quieting action. 


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